The phrase "since where" is not correct and should not be used in written English.
You could use other phrases such as "since when," "since what time," or "as of when" instead. For example, you could say, "Since when did you become interested in the stock market?".
"He has suffered similar episodes ever since, where he would pass out after spontaneous, unrestrained, heavy laughter".
We've been on holidays since where we've spent lots of money and done amazing things, but that Turkey trip would be up there as my best.
She has stayed in the Top 5 ever since, where she is accompanied by Ina Garten of "Barefoot Contessa" fame and the chef Thomas Keller.
My All American is the fourth low-budget wide-release faith-based picture set on the gridiron since Where the Game Stands Tall in August 2014.
Since where we lived had movie theaters and grocery stores, it was only when we traveled to Canal Street to department stores that segregation was most noticeable.
It was argued by many early brain researchers that if this wiring was largely random the neurons wouldn't have to know, since where an individual neuron went would not matter much.
A lot has happened, obviously, since 1900, where Moorehead's tale stops.
This is the first election since independence where you have keen contestation and equally matched contestants.
Rafter was playing in his first final since Wimbledon, where he lost to Pete Sampras.
Because of a knee injury, Williams had not played since Wimbledon, where she had left meekly in the quarterfinals.
In 11 of 15 cases since 1500 where a rising power emerged to challenge a ruling power, war occurred.
